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Tracking attosecond wave packets with extreme ultraviolet pulses

In ultrafast spectroscopy sequences of light pulses are used to study photoinduced dynamics in atoms, molecules, clusters and solids. Numerous chemical reactions, for example breaking of bonds in molecules, are induced by photon absorption. In the first instant, it’s the distribution of the electrons in the atomic shell that changes, thereby triggering the reaction. The time scales associated with these electronic processes reach into the attosecond regime (1 as = 10-18 s). Conventional spectroscopy techniques using visible laser pulses are not fast enough to resolve these processes. Therefore, researchers around the world are currently developing new laser sources and spectroscopic techniques using pulses of extreme ultraviolet or even X-ray radiation.
An international collaboration headed by Prof. Dr. Frank Stienkemeier, Dr. Lukas Bruder and Andreas Wituschek from the Institute of Physics at the University of Freiburg has succeeded in observing the ultrafast wave packet evolution induced by the coherent excitation of an electron out of an inner shell in argon atoms. The measured quantum interference pattern reflects the time evolution of the coherent superposition between excited and electronic ground state, which are separated by an energy of 28.5 eV, leading to an oscillation period of about 150 as. 
To achieve this, the collaboration has extended an ultrafast spectroscopy technique known from the visible spectral range -- coherent wave packet interferometry -- to the extreme ultraviolet spectral range: A sequence of laser pulses with well-controlled phase and timing properties was created in a highly stable interferometric setup placed in the seed laser beamline of the FEL. Subsequently the pulses were up-converted to the extreme ultraviolet spectral range, exploiting the unique coherence properties of the High Gain Harmonic Generation process at FERMI. In this way, extreme ultraviolet pulse pairs with independent and sub-cycle control of their timing and phase properties were generated.
The pulses interacted with a supersonic argon gas jet prepared at the LDM endstation. The first (pump) pulse excited a coherent superposition between the two electronic states creating a so called wave packet. The second pulse then probed the evolution after a well-defined delay. By precise variation of the delay and phases of the pulses, the researchers were able to trace the coherent evolution of the wave packet, i.e. the decoherence of the wave packet as the superexcited electron autoionizes. Fig.1 shows the level scheme of the atom under study, a schematic drawing of the experimental setup, the measurement of the time-domain interferogram of the decaying coherence and the corresponding absorption profile.
In order to isolate the weak signals caused by the evolution of the wave packet, a novel technique was used which provided the necessary high time resolution and sensitivity. To this end, phase-cycling of the extreme ultraviolet pulses was introduced to impart distinct modulation signatures on the photoproduct signals obtained in the detectors. Simultaneously, all phase jitter of the interferometric setup was recorded with a UV tracer laser. Using this as a reference for lock-in detection, a particularly high sensitivity for the weak signals was achieved while simultaneously canceling all phase jitter that would have otherwise compromised the measurements. Furthermore, the phase-resolved detection allowed for the reconstruction of amplitude and phase evolution of the wave packet, which yields the maximum information content available on a quantum system.
In conclusion, the researchers extended coherent wave packet interferometry to the extreme ultraviolet regime. For future applications, the introduced phase cycling in combination with specifically designed detection protocols is expected to provide selective information about the coupling of a specific site to an environment as well as real-time information about intra- and inter-particle decay mechanisms in the extreme ultraviolet regime.

 

Figure 1.    (a) Argon level scheme with all relevant levels and transitions (b) Experimental setup. Acousto-optical modulators (AOMs) and a delay line (DL) are used to gain independent control over the relative phase Φ21 and the delay τ, respectively. The upconverted XUV pulses propagate to the experimental chamber which is under ultra-high vacuum (UHV) and interact with the atomic beam. Ions are detected with an ion time-of-flight spectrometer (iTOF) (c) Time-domain interferogram of the decaying coherence (d) Fourier transform of (c) showing the Fano profile and the comparison to literature.
 

 

This research was conducted by the following research team:

Andreas Wituschek1, Lukas Bruder1, Ulrich Bangert1, Marcel Binz1, Rupert Michiels1, Giuseppe Sansone1, Daniel Uhl1, Frank Stienkemeier1Enrico Allaria2, Roberto Borghes2, Carlo Callegari2, Paolo Cinquegrana2, Luca Giannessi2, Miltcho Danailov2, Alexander Demidovich2, Michele Di Fraia2, Najmeh Sadat Mirian2, Ivaylo Nikolov2, Finn H. O’Shea2, Giuseppe Penco2, Oksana Plekan2, Kevin Charles Prince2, Primož Rebernik Ribič2, Paolo Sigalotti2, Simone Spampinati2, Carlo Spezzani2; Giulio Cerullo3,Marcel Drabbels4Raimund Feifel5, Richard James Squibb5Andreas Przystawik6, Tim Laarmann6Marcel Mudrich7Paolo Piseri8, Stefano Stranges9

 

Institute of Physics, University of Freiburg, Freiburg, Germany
Elettra-Sincrotrone Trieste S.C.p.A., Trieste, Italy
IFN-CNR and Dipartimento di Fisica, Politecnico di Milano, Milano, Italy
Laboratory of Molecular Nanodynamics, Ecole Polytechnique Fédérale Lausanne, Lausanne, Switzerland
Department of Physics, University of Gothenburg, Gothenburg, Sweden
Deutsches Elektronen-Synchrotron DESY, Hamburg, Germany, The Hamburg Centre for Ultrafast Imaging CUI, Hamburg, Germany.
Department of Physics and Astronomy, Aarhus University, Aarhus, Denmark
Universita` degli Studi di Milano, Milano, Italy
University of Rome “La Sapienza”,Roma, Italy


Contact persons:

Carlo Callegari, email: carlo.callegari@elettra.eu


 

Reference

Wituschek A., Bruder L., Allaria E., Bangert U., Binz M., Borghes R., Callegari C., Cerullo G., Cinquegrana P., Giannessi L., Danailov M., Demidovich A., Di Fraia M., Drabbels M., Feifel R., Laarmann T., Michiels R., Mirian N.S., Mudrich M., Nikolov I., O’Shea FH., Penco G., Piseri P., Plekan O., Prince K.C., Przystawik A., Ribič P.R., Sansone G., Sigalotti P., Spampinati S., Spezzani C., Squibb R.J., Stranges S., Uhl D. and Stienkemeier F.: 
“Tracking attosecond electronic coherences using phase-manipulated extreme ultraviolet pulses”. 
Nature Communications 11, 883 (2020), DOI: 10.1038/s41467-020-14721-2
Last Updated on Monday, 23 March 2020 10:54